There has developed in the polymer field, and especially in the packaging and related industries, a need for thermoplastic polymers that can be formed into transparent articles having high impact strength with good environmental stress crack resistance. There are needs for polymers which are useful as single components as well as for use in blends to improve properties of articles made from other commonly used polymers. The polymers satisfying these needs should be suitable for use with conventional extrusion, injection and blow molding equipment and also suitable for use in other methods of forming plastics into containers, tubes, films, fibers, and the like. Polystyrene, high impact polystyrene, branched block copolymers, and the like have been developed to meet these criteria with various degrees of satisfaction.
Much effort has been directed to the preparation of substantially transparent block copolymer resins with a variety of block structures produced by a variety of monomer addition sequences and a variety of coupling agents.
Polymerization of styrene and butadiene with organolithium initiators to produce block copolymers in which one or more non-elastomeric polymer blocks are bonded to one or more elastomeric polymer blocks has been disclosed. Block copolymers have been formed by sequential charges of the monomers, sometimes followed by introduction of one or more coupling agents into the reaction. Tapered blocks in styrene/butadiene block copolymers have been prepared by simultaneous charging of monomers to the polymerization zone. Literally thousands of various combinations of sequences of charges of monomers and coupling agents are possible.
One problem encountered in making monovinylaromatic-conjugated diene block copolymers is control of the exothermic reaction temperatures and pressure which can rapidly reach high peaks if large monomer charges are made in order to produce large blocks in the copolymers.